Low-pressure discharge lamp having an envelope encompassing the discharge space and consisting inter alia of a support

ABSTRACT

A low-pressure glow discharge lamp having an envelope encompassing the discharge space formed from a ceramic dielectric material which is coated by layers of electrical conducting material to function as a self-ballasting capacitor for stabilizing the electric discharge in the lamp.

United States Patent Willemsen LOW-PRESSURE DISCHARGE LAMP HAVING ANENVELOPE ENCOMPASSING THE DISCHARGE SPACE AND CONSISTING INTER ALIA OF ASUPPORT Inventor: Petrus Johannes Marie Willemsen, Em-

masingel, Eindhoven, Netherlands Assignee: U.S. Philips Corporation, NewYork, N.Y.

Filed: Sept. 16, 1969 Appl. No.: 858,289

Foreign Application Priority Data Sept 19, 1968 Germany ..G 67 53 632US. Cl ..313/221, 313/201, 313/210,

313/217, 313/220 Int. Cl. ..II0lj 17/16 FieldofSearch..313/201,210,217,220,109,

[ Mar. 14, 1972 Primary Examiner-Raymond F. I-Iossfeld Att0rneyFrank R.Trifari [57] ABSTRACT A low-pressure glow discharge lamp having anenvelope encompassing the discharge space formed from a ceramicdielectric material which is coated by layers of electrical conductingmaterial to function as a self-ballasting capacitor for stabilizing theelectric discharge in the lamp.

10 Claims, 3 Drawing Figures PATENTEBMAR 14 I972 INVENTOR. PETRUSJ.M.WILLEMSEN ACE/VT LOW-PRESSURE DISCHARGE LAMP HAVING AN ENVELOPEENCOMPASSING THE DISCHARGE SPACE AND CONSISTING INTER ALIA OF A SUPPORTThe invention relates to a low-pressure discharge lamp having anenvelope encompassing the discharge space and consisting inter alia of asupport in which at least a portion of the envelope comprises a layer oftransparent ceramic material and a transparent electrically conductivelayer, the last-mentioned layer being provided on the side of theceramic layer remote from the discharge, the transparent electricallyconductive layer being a current supply conductor of the lamp.

In a known lamp of the kind described, the transparent electricallyconductive layer is formed by a capacitor plate, whose ceramic layer isthe dielectric, which capacitor serves to stabilize the discharge in thelamp. An advantage of this known lamp which has been described, forexample, in Us. Pat. specification No. 2,654,042 is that a separateseries capacitor is not required.

A drawback of this known lamp is that the envelope encompassing thedischarge space is provided with a glass support. This glass makes theenvelope of the known lamp complicated. In fact, both the transparentelectrically conductive layers and the ceramic layer are still presenton the internal side of the envelope of the known lamp.

Although it is known from another embodiment of the above-mentioned US.Pat. specification to obtain a simple structure of the envelope by usingthe glass support as a dielectric of the ballasting capacitor, thissolution has the drawback that the required capacitance of the capacitorcan sometimes only be obtained when using a very thin glass sup port orwhen using a high frequency of the electrical supply voltage of thelamp. A thin glass support of course gives rise to a very vulnerablelamp and the use of a high frequency causes a complication in the supplycircuit of the lamp.

It is an object of the present invention to provide a discharge lampwhich has both a simple but robust structure of the envelope and has acapacitor impedance which is satisfactory for stabilizing when being fedby a normal low-frequency alternating voltage supply.

A low-pressure discharge lamp according to the invention having anenvelope encompassing the discharge space and consisting inter alia of asupport and in which at least a portion of this envelope comprises alayer of transparent ceramic material and a transparent electricallyconductive layer, the last-mentioned layer being provided on the side ofthe ceramic layer remote from the discharge, and the transparentelectrically conductive layer being a current supply conductor of thelamp is characterized in that the lamp is a glow discharge lamp and thatin a cross section of the envelope at the area of the ceramic layer thesupport of the envelope is constituted by said layer.

An advantage of this lamp is that the ceramic layer performs twofunctionsv The ceramic layer serves as a dielectric for the stabilizingcapacitor, but also as a supporting portion of the envelope. The lattermeans that the mechanical strength of the envelope is mainly determinedby the mechanical strength of the ceramic layer. Such a combination ofthe two functions of the ceramic layer was found to be possible in glowdischarge lamps. On the one hand this has to be ascribed to thecomparatively small values of the required series capacitances for glowdischarge lamps, and on the other hand to the comparatively smalldimensions of these lamps, and this because the use of this ceramicmaterial makes it possible to obtain both a sufficiently high ballastingimpedance of the capacitor and a still acceptable light transmission dueto the small envelope section required on account of mechanicalconsiderations.

In a lamp according to the invention the other electrode of the seriescapacitor is formed for example, by ionized gas in the discharge spaceduring operation of the lamp.

In a lamp according to the invention the envelope on the side of theceramic layer facing the discharge is preferably provided with a secondtransparent electrically conductive layer which is in contact with thedischarge space.

An advantage thereof is that the ignition voltage of the lamp may becomparatively small, because the second transparent electricallyconducive layer may now function as the lamp electrode which may have awork function which is low for electrons.

In a further advantageous embodiment of a glow discharge lamp accordingto the invention the lamp is oblong and is provided with a penlikeelectrode the longitudinal axis of which substantially coincides withthe longitudinal axis of the lamp.

An advantage of this embodiment is that the glow discharge lamp in thiscase may have a light distribution wherein the light intensity in a flatplane at right angles to the longitudinal axis of the lamp issubstantially the same in any direction.

In a further embodiment, the lamp is oblong, the envelope near one endof the lamp consisting of glass.

An advantage of this embodiment is that during the manufacturing processthe ends of the lamp can easily be sealed in a vacuumtight manner. Infact, this is a simple matter when using glass.

It is feasible that only one end of the lamp is made of glass, forexample, an end to which an exhaust tube was secured. It is alsofeasible that both ends of the lamp are provided with a glass part ofthe envelope. One of these parts or both parts consist, for example, ofglass beads.

In a further preferred embodiment according to the invention the lampincludes an envelope having both a second transparent electricallyconductive layer and a third transparent electrically conductive layer,the two coatings being in contact with the discharge space, and theelectric resistance of the envelop between the third and the secondtransparent electrically conductive layer being highly resistive, thedischarge current of the lamp flowing through the third electricallyconductive layer. A highly resistive electric resistance is understoodto mean a resistance in the order of that of an isolator.

An advantage of this lamp is that the light radiation in this case issubstantially not influenced by the lamp electrodes.

The third electrically conductive layer may be connected, for example,to a connection wire which protrudes through the envelope of thedischarge space.

A lamp according to the invention preferably has not only the saidfirst, second and third transparent electrically conductive layers, butalso a fourth transparent electrically conductive layer, which fourthlayer is provided on the side of the ceramic layer remote from thedischarge, all this in such a manner that the electric resistance of theenvelope between the fourth electrically conductive layer and the firstelectrically conductive layer is highly resistive, the fourthelectrically conductive layer being a current supply conductor of thelamp, the second conductive layer facing the first conductive layer andthe third conducive layer facing the fourth conductive layer.

In this case connection wires through the envelope of the dischargespace are superfluous. In fact, the lamp is now fed through the firstand the fourth transparent electric conductor. Going from the firsttransparent conductor the current passes successively a first seriescapacitor, the discharge path and a second series capacitor. Then thecurrent has reached the fourth transparent electric conductor.

In certain cases, for example, when the lamp is incorporated in apermanently closed housing of, for example, synthetic plastic material,a transparent electric conductor may form the exterior of the lamp.

The exterior of the envelope preferably consists of a coating of atransparent insulation material, particularly a transparent lacquercoating.

An advantage of the last-mentioned embodiment is that the lamp cansafely be touched by hand.

In order that the invention may be readily carried into effect, a fewembodiments thereof will now be described in detail by way of example,with reference to the accompanying diagrammatic drawing in which:

FIG. I is a longitudinal section of a glow discharge lamp according tothe invention;

FIG. 2 is a perspective view of a second glow discharge lamp accordingto the invention;

FIG. 3 is a longitudinal section of the lamp of FIG. 2.

In FIG. 1, reference numerals l and 2 denote connection terminals, whichare intended to be connected to an alternating voltage supply of 220 v.,50 Hz. An electric conductor 4 is connected between terminal I and aglow discharge lamp 3. An electric conductor 5 is connected between theterminal 2 and the glow discharge lamp 3. The lamp 3 comprises a lighttransmitting cylindrical part 6, an endpart 7 in the form of ahemispherical bead and an endpart 8 having an arclike profile. The glowdischarge lamp has a length of approximately 3 cm. The cylindricalenvelope 6 consists of an external electrically conductive transparentlayer 9 which is provided on the outer side of the envelope 6. In thiscase this layer consists of tin oxide, but it is feasible that adifferent material, for example, indium oxide is used for this purpose.The envelope 6 further includes a layer of a ceramic material 10. Thismaterial mainly consists of barium titanate. The interior of theenvelope 6 includes a second transparent electric layer 11 whichlikewise consists of tin oxide. In FIG. I the layers 9 and 11 are showntoo thickly, namely to clearly indicate their location. Actually, theirthickness is at the most a few percents of that of the thickness of theceramic layer 10. The glow discharge lamp 3 further includes a penlikeelectrode 12 which is provided on the longitudinal axis of the lamp. Thehemispherical head 7 and the arclike endpiece 8 consist of glass, namelya soft glass the coefficient of expansion of which is substantiallyequal to that of the ceramic material of the layer 10. The arclikeportion 8 is the remainder of a pinched glass exhaust tube through whichthis lamp was exhausted and filled with neon gas during manufacture. Theelectric conductor 4 is connected to the electrically conductive layer9, for example, by means of a soldered joint. The electric conductor 5is connected between terminal 2 through the glass bead 7 to the penlikeelectrode 12. The lamp 3 if filled with neon gas, whose pressure wasapproximately 30 torr. The external diameter of the lamp wasapproximately 3 mm. and the internal diameter was approximately 2 mm.,as already previously stated the length was approximately 3 cm. Thelength of the cylindrical part 6 was approximately 2 5; cm.

In the lamp described the combination of the conductive layer 9, theceramic layer 10 and the conductive layer 11 constitutes a seriescapacitor for the discharge which takes place in the discharge spaceencompassed by the envelope of the lamp. This combination of threelayers also constitutes the cylindrical portion of the envelope of thesaid lamp. The electric current flows via the connection terminal Ithrough the electric conductor 4 to the capacitor plate 9. The layer 11,which is the other plate of the capacitor, also functions as anelectrode of the lamp 3. A current flows from this electrode 11 throughthe discharge space to the electrode 12 and thence through the electricconductor 5 to the connection terminal 2. The lamp described is intendedto be used in a sealed housing (not shown) of a transparent insulatingmaterial.

The capacitance of the capacitor of FIG. 1 may approximately bedetermined by way of the formula:

C the capacitance in F arad; E, the relative dielectric constant of theintermediate layer 10;

l= the length of the cylinder 6 in meter; and

D D, the ratio between the external diameter of the lamp and theinternal diameter of the lamp.

In the relevant case the relative dielectric constant of the ceramicintermediate material was approximately 4,000. Alter substitution ofl=2.5.10 D JD, 1%, the capacitance C of the capacitor is found to beapproximately 14,000 pf. A comparatively small resistivity is added tothis capacitance, which resistivity is formed by the electric resistanceof the transparent electrically conductive coatings.

C E of a cylindrical capacitor; wherein:

In the case of the glow discharge lamp described the lamp current wasapproximately 1 ma. A capacitance of l4,000 pf. is a satisfactorycapacitance for the above-mentioned current source to which the lamp isconnected.

The mechanical strength of the cylindrical envelope 6 is substantiallydetermined by the mechanical strength of the ceramic layer 10.

In FIG. 2 the reference numeral 30 again denotes a glow discharge lamp.This glow discharge lamp has a light transmitting cylindrical portion 31which is closed on either side with the aid of glass beads 32 and 33,respectively. The cylindrical portion 31 consists of a ceramic support34 (see also FIG. 3) half the circumference of which is provided withtwo externally located transparent electric conductors (35, 36). Asemicylindrical transparent electric conductor 37 is provided internallyof the ceramic layer 34. The conductor 37 faces the conductor 35 (seealso FIG. 3). A likewise semicylindrical electrically conductive layer38 is provided, likewise as the layer 37, internally of the ceramiclayer 34. The layer 38 faces the transparent electrically conductivelayer 36. The conductor 35 is connected to a connecting terminal 39, theconductor 36 is connected to a connecting terminal 40. These connectionterminals 39 and 40 are intended to be connected to an alternatingvoltage mains of 220 v., 50 Hz. The layers 35 and 36 of the glowdischarge lamp 30 are covered by a transparent lacquer coating 4].

If desired, the lacquer coating may, however, partly consist of areflective material. THe material of the ceramic layer 34 is the same asthat of the layer of the lamp of FIG. 1. The material of the transparentconductors is the same as that of the layers 9 and 11 of the lamp ofFIG. 1.

In FIG. 3 the locations of the different layers such as the ceramiclayer 34 and the electrically conductive layers 35 to 38 inclusive areshown in a longitudinal section of the lamp. The coating 41 and thelayers 35 to 38 inclusive are shown comparatively too thickly in FIGS. 2and 3. This has been done for the same reason as stated in thedescription of the layers 9 and 11 of FIG. 1.

The lamp of FIGS. 2 and 3 also had a length of approximately 3 cm., anexternal diameter of 3 mm. and an internal diameter of 2 mm. The overalllength of the semicylindrical conductors 37 and 38 was approximately 2%cm. In the relevant case the lamp current was approximately one-eighthma. In this case the stabilization of the discharge current was carriedout by two capacitors, to wit the capacitor including the plates 35 and37 and the capacitor including the plates 38 and 36. In fact, thesecapacitors are incorporated in the series circuit 39, 35, 37, thedischarge path 38. 36, 40. The capacitance of these two capacitorscombined is approximately one-eighth of that of the lamp of FIG. 1. Withthe above-mentioned current source and the above-mentioned lamp currentintensity of the glow discharge lamp 30 this also gives a satisfactorystabilization of the glow discharge.

It is feasible that a cylindrical portion of the lamp envelope is notclosed with the aid of a glass bead, but by means ofa different closure,for example, a flat thin-walled final portion which is transverse to thelongitudinal axis of the lamp.

What is claimed is:

I. A discharge lamp comprising an oblong envelope forming a dischargespace, end parts for hermetically sealing said envelope, said envelopeessentially consisting of a transparent ceramic dielectric materialhaving a coefficient of expansion substantially equal to that of saidend parts, an ionizing gas filling said discharge space, electrodes forproducing a gaseous discharge within said envelope, electricallyconductive transparent layers on opposite sides of said envelope inconfronting relationship, the inner conduction layer functioning as oneof said electrodes in said discharge space, the inner and outerconducting layers and said dielectric material forming a ballastingcapacitor in series with said discharge space for stabilizing saidgaseous discharge within said lamp, an electrode functioning as thesecond of said electrodes in said discharge space, and terminal meansfor providing current to said lamp.

2. A discharge lamp as claimed in claim 1 wherein said second electrodeis positioned along the longitudinal axis of said lamp.

3. A discharge lamp as claimed in claim 1 wherein said transparentceramic dielectric material essentially consists of barium titanate andsaid electrically conducting material essentially consists of tin oxide.

4. A discharge lamp as claimed in claim 1 further comprising a lacquercoating of a transparent insulating material for said electricallyconducting material.

5. A discharge lamp as claimed in claim 4 wherein said lacquer coatingpartly consists of a reflective material.

6. A discharge lamp as claimed in claim 1 wherein said end partsessentially consist of glass, one of said end parts being in the form ofa hemispherical bead and the other having an ar clike profile.

7. A discharge lamp as claimed in claim 1 wherein said end parts areflat thin walled portions positioned transverse to the longitudinal axisof said lamp.

8. A discharge lamp as claimed in claim 1 wherein said internalelectrically conductive, transparent layer has a low work function.

9. A discharge lamp comprising an oblong envelope forming a dischargespace, end parts for hermetically sealing said envelope, said envelopeessentially consisting of a transparent ceramic dielectric materialhaving a coefficient of expansion substantially equal to that of saidend parts, an ionizing gas filling said discharge space, at least onepair of electrically conductive transparent layers spaced apart onopposite sides of said envelope in confronting relationships, theinternal conducting layer functioning as an electrode for producing agaseous discharge, the inner and outer conducting layers and saiddielectric material forming a ballasting capacitor in series with saiddischarge space to stabilize said gaseous discharge within said lamp,and terminal means connected to said external conducting layers forproviding current to said lamp.

10. A discharge lamp as claimed in claim 9 wherein said electricallyconductive transparent layers have a semicylindrical shape.

mg mm'mn STATES PATIENT owner:

CERTHEFMATE @F QQRREQTWN Patent No. 3649864 Dated May 4, 1972 Inventor)PETRUS JOHANNES MARIE WILLEMSEN It is certified that error appears inthe above-identified patent and that said Letters Patent are herebycorrected as shown below:

Column 2, line 18, "vacumntight" should read vacuum-tight Column 3, in"the formula, "10' 1' should read 10' -1 Column -1 line 36, after layerinsert l0 Signed and sealed this ZLLth day of October? 1972.

(SEAL) Attest:

EDWARD MQFLE'ZGIER JRO ROBERT GOTTSCHALK Attesting Officer Commissionerof Patents

1. A discharge lamp comprising an oblong envelope forming a dischargespace, end parts for hermetically sealing said envelope, said envelopeessentially consisting of a transparent ceramic dielectric materialhaving a coefficient of expansion substantially equal to that of saidend parts, an ionizing gas filling said discharge space, electrodes forproducing a gaseous discharge within said envelope, electricallyconductive transparent layers on opposite sides of said envelope inconfronting relationship, the inner conduction layer functioning as oneof said electrodes in said discharge space, the inner and outerconducting layers and said dielectric material forming a ballastingcapacitor in series with said discharge space for stabilizing saidgaseous discharge within said lamp, an electrode functioning as thesecond of said electrodes in said discharge space, and terminal meansfor providing current to said lamp.
 2. A discharge lamp as claimed inclaim 1 wherein said second electrode is positioned along thelongitudinal axis of said lamp.
 3. A discharge lamp as claimed in claim1 wherein said transparent ceramic dielectric material essentiallyconsists of barium titanate and said electrically conducting materialessentially consists of tin oxide.
 4. A discharge lamp as claimed inclaim 1 further comprising a lacquer coating of a transparent insulatingmaterial for said electrically conducting material.
 5. A discharge lampas claimed in claim 4 wherein said lacquer coating partly consists of areflective material.
 6. A discharge lamp as claimed in claim 1 whereinsaid end parts essentially consist of glass, one of said end parts beingin the form of a hemispherical bead and the other having an arclikeprofile.
 7. A discharge lamp as claimed in claim 1 wherein said endparts are flat thin walled portions positioned transverse to thelongitudinal axis of said lamp.
 8. A discharge lamp as claimed in claim1 wherein said internal electrically conductive, transparent layer has alow work function.
 9. A discharge lamp comprising an oblong envelopeforming a discharge space, end parts for hermetically sealing saidenvelope, said envelope essentially consisting of a transparent ceramicdielectric material having a coefficient of expansion substantiallyequal to that of said end parts, an ionizing gas filling said dischargespace, at least one pair of electrically conductive transparent layersspaced apart on opposite sides of said envelope in confrontingrelationships, the internal conducting layer functioning as an electrodefor producing a gaseous discharge, the inner and outer conducting layersand said dielectric material forming a ballasting capacitor in serieswith said discharge space to stabilize said gaseous discharge withinsaid lamp, and terminal means connected to said external conductinglayers for providing current to said lamp.
 10. A discharge lamp asclaimed in claim 9 wherein said electrically conductive transparentlayErs have a semicylindrical shape.